The generation of microwaves using electron tubes, in particular using a free electron maser, gyrotron or klystron, is generally known. As an example, a gyrotron includes an electron source for generating a hollow beam of highly accelerated electrons and a cryomagnet resonance device for forcing the electrons into a cyclotron motion, wherein the microwave is emitted. A beam collector is provided for collecting the electron beam after separation of the microwave with a microwave optic. The beam collector is adapted not only for absorbing the electric current represented by the electron beam, but rather for dissipating waste power, which has been kept in the electron beam after the microwave emission.
Heat dissipation in beam collectors represents a serious problem in particular with high power microwave generators. As an example, high power millimeter wave vacuum tubes operate with a radio frequency (rf) power of typically 1 MW in cw-mode with an efficiency of 30% to 50%. In this range of efficiencies, typically 1 to 2 MW power remains in the electron beam after the microwave generation. This remaining power must be dissipated as waste power in the beam collector. The beam collector typically is made from copper with a cylindrical shape. Electrons are guided by an axis symmetric strong stationary magnetic field (typically 5-6 T) through an entrance area into the axis-symmetric collector. The diverging magnetic field lines and thus the drifting electrons intersect at some vertical position with the collector wall. The intersection area (strike area) forms a horizontal ring with a typical power density of e.g. 20 MW/m2. Although copper as excellent cooling properties and a sophisticated water-cooling system is integrated into the beam collector wall, this power density is far beyond existing cooling technology. With a continuous operation, this power density would lead to melting of the beam collector.
For avoiding damage to the beam collector, available gyrotrons are adapted for a collector sweeping technique (magnetic field sweeping technique, see e.g. S. Alberti et al. “European high-power CW gyrotron development for ECRH systems” in “Fusion Engineering and Design” vol. 53, 2001, p. 387-397). Generally, collector sweeping comprises superimposing the stationary diverging magnetic field with a magnetic sweeping field, which sweeps (continuously moves, deflects) the hollow electron beam over the inner wall of the beam collector to reduce the local power density in a time average (FIGS. 5A and 5B).
In particular, FIGS. 5A and 5B illustrate a cylindrical beam collector 230′ of a conventional gyrotron (not completely shown). The hollow electron beam 1′ is directed to the beam collector 230′ along the longitudinal (axial) extension thereof (parallel to the positive z-direction). With the diverging magnetic field 2′, the electron beam 1′ is directed to the inner walls of the beam collector 230′. Without sweeping, the intersection area 3′ formed by the electron beam 1′ with the inner wall of the beam collector 230′ would be a circular area as shown with the central dotted ring in FIG. 5A.
According to FIG. 5A, collector sweeping is provided by a vertical sweeping coil 22′ surrounding the outer wall of the beam collector 230′ and extending along the longitudinal extension thereof. With the vertical sweeping coil 22′, a vertical sweeping field is created adding a periodically alternating axial vector component (z-component) to the diverging magnetic field (Vertical Field Sweeping System, hereinafter “VFSS”). As a result, the electron beam 1′ is swept along the inner wall of the beam collector 230′. The intersection area 3′ formed by the electron beam 1′ is a shifting circular area. Dashed rings in FIG. 5A mark the upper and lower turning points 4′ of the deflected the electron beam 1′.
The VFSS has a general disadvantage in terms of low electrical efficiency. The copper wall of the beam collector 230′ represents a single turn, short-circuited coil efficiently shielding the vertical sweeping field. Powerful AC-power supplies in connection with large, water cooled sweep coils are required to provide the necessary sweeping capability. This disadvantage can be avoided with the conventional collector sweeping method illustrated in FIG. 5B (Transverse Field Sweep System, hereinafter “TFSS”).
With TFSS, collector sweeping is provided by transversal sweeping coils 11′. In this case, a transversal sweeping field is created adding a rotating horizontal vector component to the diverging magnetic field. With the transversal sweeping field, the intersection area of the electron beam 1′ is a rotating ellipse. As a small distortion perpendicular to the z-direction is enough for an efficient deflection of the electron beam, the transversal sweeping coils 11′ can be positioned in front of an entrance area of the beam collector 230′ so that the above shielding problem of the VFSS technique is significantly reduced. Furthermore, this section of the gyrotron is built from stainless steel rather than copper with reduced conductivity.
A general problem of both VFSS and TFSS techniques is related to the so-called power peaking during the sweeping period. FIG. 6 illustrates vertical power density profiles of the collector temperature increase for the VFSS technique (dots) and the TFSS technique (dashes). While the vertical sweeping results in two power peaks at the turning points 4′ (FIG. 5A) of the sweeping period, the transversal sweeping shows a single power peak at the lower limitation (near the entrance area of the beam collector). The power peaking represents a main disadvantage, because the power density in the maximum of the power distribution determines the overall collector capability.
Generally, there is an interest to reduce the power peaking not only in gyrotron beam collectors, but rather in any beam collector of a high-power vacuum device, as it is applied e.g. in other microwave generators, in particular for the purpose of heating a plasma in a fusion reactor.
It could therefore be helpful to provide improved collector sweeping methods and apparatuses for controlling an electron beam in a beam collector avoiding the disadvantages and restrictions of the conventional techniques.